[swift-evolution] [swift-evolution-announce] [Review #3] SE-0117: Allow distinguishing between public access and public overridability

Dave Abrahams dabrahams at apple.com
Thu Jul 21 15:13:36 CDT 2016

on Thu Jul 21 2016, Matthew Johnson <swift-evolution at swift.org> wrote:

>> On Jul 21, 2016, at 12:47 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> on Thu Jul 21 2016, Matthew Johnson <swift-evolution at swift.org> wrote:
>>>> 	* What is your evaluation of the proposal?
>>> +1 to the first design.  I think this is a great solution that
>>> balances the many considerations that have been raised on all sides of
>>> this issue.  `open` is 2 characters shorter than `public` so
>>> complaints about boilerplate are no longer valid.  `internal` is the
>>> “default” - neither `public` nor `open` are privileged as a “default”
>>> for publishing API outside of a module.
>>> I am interested in language enhancements such as exhaustive pattern
>>> matching on classes and protocols which rely on knowledge of the full
>>> class hierarchy.  Such enhancements will be far more useful if the
>>> language supports non-open, non-final classes.
>>> There are design techniques that would require additional boilerplate
>>> if we cannot have non-open, non-final classes.
>>> Most importantly, requiring library authors to choose `public` or
>>> `open` provides important documentation value.  Users of the library
>>> will know whether the author intends to support subclasses or not.
>> I think this reasoning is flawed.
>> If you make any methods overridable outside your module (“open”),
>> obviously you mean to allow subclassing outside the module.  If you have
>> no open methods, there's absolutely nothing you need to do to “support
>> subclasses,” and from a design point-of-view, there's no reason to
>> restrict people from subclassing.
> I disagree.  As has been discussed when a class is not open the author
> does not make a commitment to allow subclasses.  

Yes, but that argument is based on the *assumption* that disallowing
subclassing is somehow an important dimension of safety or preserving
API contracts, independent of overriding.  It isn't.

Demonstration: we don't make that argument about aggregation: you can't
prevent someone from making your public struct a stored property in a
type defined outside the module.  Judgements about programming style
aside, in the absence of overriding, subclassing and aggregation are
identical from an encapsulation point-of-view.

> The right to make the class final is reserved for the future.  Maybe
> this is the “nice point of control” you refer to and don’t find
> compelling?  I would prefer to have library authors acknowledge that
> they intend to allow subclasses and make that commitment explicit.

The question is, why?

> For me it isn’t about control as much as it is about making the API
> contract explicit and acknowledged.  I have wondered about the intent
> of library authors enough times to find this explicit statement in the
> language worthwhile.

Yeah, but we don't add language features to represent every possible
author intention, and there's a good argument that any intention that
can be violated without harm shouldn't be enforced.

> I also think language features enabled by knowing the whole class
> hierarchy will provide more value than “compositional subclasses” as
> long as we gain better support for composition elsewhere in the
> language.

I agree that there are better ways to solve the composition/API
forwarding problem.

>> The only reasons I can see for allowing people to prevent non-final
>> classes from being subclassed outside the module in which they are
>> defined are:
>> 1. It feels like a nice point of control to have.
>> 2. Marginal performance gains as noted in the proposal
>> I personally don't find these to be convincing.  #1 in particular seems
>> like a poor way to make language design decisions.  If we decide to add
>> this point of control, I'll justify it to myself in terms of #2.
>> P.S., I can live with either alternative; it's just important to me that
>> we understand the situation clearly when evaluating them.
> I agree with this.
>> HTH,
>> -- 
>> Dave
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